The patent application PCT/WO 00/69565 to G. Medoro describes an apparatus and method for manipulating and locating particles using closed cages of dielectrophoretic potential. Besides this, PCT/WO 00/69565 teaches methods of separation. A first method is based on the differential action, on populations of different cells, of dielectrophoresis (DEP) negative (NDEP) and positive (PDEP), and does not permit discriminating particles if they are both subject to NDEP or PDEP. Other methods of separation instead require using information from sensors integrated in the device, and cannot be implemented when there are no such sensors.
Many other methods of separation are known that nevertheless require using flows of liquid, which is an undesirable characteristic in many applications, since it requires generating well controlled flows. A review of these methods, above all as concerns the use of dielectrophoresis is given in Electrophoresis 2002, 23, 2569-2582 M P. Hughes, and in Electrophoresis 2002, 23, 1973-1983 P. R. C. Gascoyne, J. Vykoukal, and in the related bibliographical references.
In particular, U.S. Pat. No. 5,888,370 to Becker et al. teaches among other things to use dielectrophoresis to determine the height of particles in a speed-profile of liquid flow, to which the separation of the particles follows.
Other methods of separation based on dielectrophoresis do not require using flows of liquid, but they utilize so-called Travelling waves (TWD). These methods are based on the application to an array of electrodes of sinusoidal voltages at suitably phase-shifted radio frequencies (e.g. 0, 90, 180, 270 degrees). The result is an electric field that travels in space at a speed (VTW) equal to the distance between the electrodes with the same phase divided by the period of the single sine voltage. The phase-shift between the polarization induced on the particle and the electric field that generates it causes a translation of the particle, at a speed of orders of magnitude less than VTW, and asynchronous with it. Accordingly it is not possible to maintain an accurate control on the position of the particles after separation, which may be desirable in many applications, for instance if it is wished to determine the quantity of separated particles by placing them above sensors.
The limitations of the known art are superseded by this invention that permits separating particles even if they are subject to the same force (e.g. NDEP), it requires no flow of liquid, and it permits keeping a control on the position of the particles during and after separation or measurement.